The present invention relates generally to voltage controllers for power systems, and more particularly to a power system voltage controller used at a load-site.
Electrical power generation is often performed at generation sites distant from the consumers of electrical power. The electrical power is transmitted from the generation sites to the consumers by feeder distribution networks. Each consumer expects that the electrical power delivered over the feeder distribution networks will be at a stable voltage, and electrical machinery used by the consumers is likewise designed to operate at stable voltages. The voltage at the consumer end of the feeder distribution networks, however, is generally not constants For example, as large loads arc placed on the system, such as due to the operation of heavy industrial machinery, the voltage at the load site may vary.
Voltage regulation devices are therefore often used to perform voltage regulation at load sites. These voltage regulation devices are often in the form of tap changers. Tap changers operate by providing a series of connections at slightly different voltages. As the voltage at the load site changes, a mechanical switch modifies a mechanical connection to contact a tap at either a higher or a lower voltage. Tap changers, however, are relatively costly, and arc subject to mechanical failure. In order to avoid excessive wear on the tap changers and to prolong the service life of tap changers, shunt capacitors may also be used to regulate load site voltage. Shunt capacitors, however, may also be costly and are also subject to failure.
It is also often desirable to reduce power supplied by the utility, both real and reactive power, by providing locally generated power. Accordingly, sometimes local power generation units are used at the load site to supplement power supplied by the utility, as well as to regulate voltage at the load site.
An example power regulator system is illustrated in FIG. 3. In the system of FIG. 3, a local power source and associated inverter (indicated together) 351 are coupled to a transmission line at a load site. The power source and associated inverter provide power to the load. Coupled to the connection between the power source and associate inverter and the load is a filter including a capacitor (not shown). The local power source is therefore connected in parallel to the utility (not shown).
The power regulator system of FIG. 3 includes a current regulator (311 and 323). The current regulator provides a signal to the local power source and associated inverter for use in the control of the power source and associated inverter. In the system illustrated in FIG. 3, a current vector of the inverter is regulated to a desired value.
The current regulator is a vector control system based upon a park-vector, or space-vector, representation of all three-phase electrical quantities. The use of park-vectors facilitate transformation of control signals from sinusoidal values in a stationary frame to largely DC level signals in a synchronous frame. Methods of transforming signals from one reference frame to another is well known by those familiar with the art. Park vectors are described in, for example, Transient Phenomena in Electrical Machines by P. K. Kovacs, published by Elsevier (1984), the disclosure of which is incorporated herein by reference.
Accordingly, the inverter current output vector i.sub.inv is determined. As the inverter current output vector i.sub.inv is determined in the stationary reference frame, a capacitor voltage vector v.sub.cap is also determined for use in transforming the inverter current output vector to the synchronous frame. In order to reduce ac signal components in the synchronous frame signal, the capacitor voltage is filtered to reduce harmonics and other noise at frequencies other than those about the fundamental system frequency. Therefore, a rotational reference frame is extracted from the filtered capacitor voltage vector to form a unit vector for transformation to the synchronous frame in an extraction unit 363. The unit vector is provided to a transformation unit 332, as is the inverter current output vector i.sub.inv. The transformation unit 332 outputs a vector i.sub.k, which is comprised of essentially DC signals of a real component and a reactive component, representing the inverter current vector in the synchronous frame. The vector i.sub.k, therefore, is the inverter current output vector in the synchronous frame.
The vector i.sub.k is compared with a command reference vector i.sub.ikcmd at a summer 323. Generally the command reference signal i.sub.ikcmd is empirically determined, and is changed only infrequently. As it is often desirable to provide as much real power from a local power source generator to the load as possible, the real power component is generally set to a maximum, which is a value of one power unit (p.u.) in a normalized system. The reactive component of the command reference signal i.sub.ikcmd is generally set to 0.
The output of the summer 323 is provided to a controller 311. The controller 311, in the prior art, amplifies the output of the summer, and provides a voltage vector command in the synchronous frame. The voltage vector command provided by the controller is transformed to the stationary frame by a transformation unit 333, again based upon a unit vector provided by the extraction unit 331. The output 313 of the transformation unit is provided to the local power source and associated inverter to control inverter operation.
The control system of FIG. 3, as described above, is well known to those skilled in the art. Such a control system reduces real power required to be supplied by a utility, as well as providing voltage regulation at a load site. The system of FIG. 3, however, does not optimize provision of reactive power to the system, and does not adaptively modify local power supply output based on changes in real power requirements. Further, in the system of FIG. 3 the filter may introduce unwanted power variation, particularly about resonant frequencies of the capacitor.